Brakes - Combination Valve / Proportioning Valve

My old '71 Monte Carlo has been sitting for 4 years. Brakes were not great even before then, (low, soft pedal). Replaced Master Cylinder, front rotors & calipers, rear wheel cylinders & shoes. New flex hoses all around.
Fronts bled fine, rears only gave up about an ounce of fluid or less per side, then nothing. I am quite certain the Combination Valve has shut off flow to the rears. I cannot find a replacement valve anywhere. I've read hundreds of posts on this and other forums, many saying to press in the pin on the end of the valve, others saying to pull it out. The pin will not move either direction, even with substantial force.
I have heard & read that I can crack open one or both of the front bleeders, then hit the brakes, to center the valve. Also have read many who tried it and didn't work. I am sure I have plenty of air in my rear brake lines, so that method may not develop any rear pressure to move the valve back to center. I have read that you can remove the brake warning light switch from the combo valve, and manually slide the valve to center through that opening. But the switch does not have a hex on it, just a smooth round housing. So how would I remove it? Can't get channel-locks on it, since there are 2 brake lines next to it?
I poached the following text from an old AF post, which describes my problem, and a tool to center the valve, (which no parts store I have asked knows anything about):
"If the combination valve shows no resistance, it has shut off one of the hydraulic systems, in your case, probably the rears. Auto parts stores have a tool designed to force the valve to the center position for bleeding purposes. You can try to gravity bleed the rear brakes one at a time. When the fluid runs clear from the rear bleeders, close the bleeders, then push the brake pedal down, the valve should center itself. An alternative procedure to center the valve is to open a front bleeder and give the brake pedal a quick jab. That procedure doesn't always work, especially if there is excessive air in the rear hydraulic system. "

I believe if I could get fluid into the rear lines, I could use the method described to recenter the valve. I would be willing to replace it altogether, (looks like a downright messy job that will have brake fluid everywhere, with 5 line fitting into/out of that valve assy), but nobody sells them. And I don't know of any junkyards that have early 70's cars sitting around... Really don't think mine is bad, just need to find a method to re-center the shuttle valve that blocked fluid flow to the rear side.

As always, any advice or information will be greatly appreciated!

The combo valve should have a " button" on the end, maybe covered by a small rubber bellows, hold the valve out with maybe needle nose pliers ( moves about 1/8"), this opens the flow to the rear brakes.

The combo valve should have a " button" on the end, maybe covered by a small rubber bellows, hold the valve out with maybe needle nose pliers ( moves about 1/8"), this opens the flow to the rear brakes.
It does have a button, it is smooth, no grooves or any features. Many posts I've seen on several forums say it must be held "in" to permit bleeding of the rear brakes. Some others say to pull it outward to do the same. But all of them say that button will not re-center the valve, it will only "temporarily" allow bleeding (fluid flow) to the rear brakes. (Or fronts, depending on which side of system the valve sensed a leak/low pressure and blocked off).

I tried pushing it in, and pulling it out with Channel-Lock pliers. It does not move in either direction. I don't know if it is frozen, but the old fluid I drained from the Master Cyl and calipers/wheel cylinders did not look bad - it was yellow-ish, but not rusty or dark. (I replaced all of the same parts 14 years ago, and it has not had many "miles" put on it - car is used only for bracket/drag racing).

This afternoon I may try the method listed in my original post - Open front bleeders and stab the pedal. This would theoretically create less pressure in the front system than the rear, (the combo valve would sense the pressure difference, and interpret this as a "failure" in the front side), so it would move the shuttle valve to block that side, thereby opening the rear side to flow from the M.C.):runaround:
If I could get this to happen, I could gravity fill the rear side, bleed it, then re-bleed the fronts. When all corners are properly bled, supposedly one hard stomp on the brakes will recenter the valve...
Again, any comments will be welcomed.:wink:

Check this link.https://www.google.com/search?q=gm+combination+valve&biw=1024&bih=677&tbm=isch&imgil=5zi0PO8F7TxQPM%253A%253Bhttps%253A%252F%252F encrypted-tbn0.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9 GcTR0TcLtDjRCLYtDl-uPkp9vW8XY40SAfw3D2RQkp0sDSADuXdb%253B550%253B222% 253BhJlyNq9tfv4mPM%253Bhttp%25253A%25252F%25252Fww w.performanceonline.com%25252FGM-PROPORTIONING-and-COMBINATION-VALVES-AC-DELCO-172-1353-and-172-1361-TYPE-CHROMED-19296%25252F&source=iu&usg=__ZlsYWt-Sn2AEy24ADr4CgHQ1Vr4%3D&sa=X&ei=bf5nU73mCvGlsASylYHAAg&ved=0CE0Q9QEwAQ#facrc=_&imgdii=_&imgrc=afdhi5zoY2n2_M%253A%3BrcAyBCUt8E9LpM%3Bhttp% 253A%252F%252Fwww.rowand.net%252FShop%252FTech%252 Fimages%252FGMCombinationValve.jpg%3Bhttp%253A%252 F%252Fwww.rowand.net%252FShop%252FTech%252F1958Bui ckDiscBrakeSwap.htm%3B1396%3B1018

Max Wedge, thank you for the tip. Almost all the info I could find in repair manuals said the pin needs to be held "IN" to allow fluid flow to the rear side. After reading your post, I cleaned the unit, applied a bit of PB Blaster, (the BEST penetrating oil I have found for loosening old fasteners & brake lines), and eventually got the pin to move "outward" about 1/8". Worked in & out, then left it out, opened the rear bleeder fittings, and got fluid to flow using gravity.:cool: Still don't have a firm pedal, but at least it doesn't go to the floor anymore. I need to find an assistant to help me bleed all 4 corners again. My hand-held vacuum pump is not getting it done, sucks too much air around the threads of the poorly-fitting bleeder screws on these @#$%^&! Chinese Autozone wheel cylinders... :banghead:

Also, I suspect the valve might still off-center, and possibly/partially blocking flow to the rears, I need to get the back end up in the air on stands and see if the rear brakes are working at all.

Maybe time for a new one?

Maybe time for a new one?

New car?:biggrin: Actually, I met a new neighbor last night, he was walking his dog as I was attempting to again bleed the rears with my vacuum pump.
He offered to help, so I put him on the brake pedal and bled all 4 corners the old-fashioned way. Got more air out of the system, and now have a firm pedal. (My thinking now, says the 15 or 20 inches of vacuum from a hand pump is not nearly enough force to produce the kind of flow you get with pedal pressure). :grinno:
So I have pedal, drove it around the block, made about 15 somewhat hard stops. The drums out back got equally good & warm, but not hot, so I think the shoes are adjusted about right.
So now the hydraulic system seems good, but the amount of pedal effort it takes to stop the car is still almost ridiculous. I cannot get it to lock up any of the tires. I am almost positive the booster is dead. I tested it the way I have seen and done in the past - pedal down, start the engine, and pedal should move downward a bit, due to the vacuum/power assist. Pedal doesn't move. Feels no different running or not. What other test(s) can I perform on it? Also, my engine may not develop enough vacuum to get full power assist, but should still see some benefit? (Cam is solid/mechanical, flat-tappet, lift is .637", duration is 270* @ .050"). Vacuum at idle is only 7-8 inches. But I think I should still feel some power braking effect. At least on deceleration, when vacuum is highest. I left it in low gear, accelerated up to about 4,000 RPM, then let off the gas and hit the brakes, thinking it should have been at least 12-15" of vacuum. Made no difference at all.

Any booster testing tips, or confirmation that is must be no good?
Thanks again, I appreciate your help, we are getting there! (It is good to be able to stop the car!):grinyes::cool:

pull the hose out of the booster and see if you have vacuum at the valve on the end while its idling. if you have vaccuum then your booster is probably bad

I put a vacuum gauge on the manifold port, and at idle, it only creates 3-5" of vac @ 1200 RPM. Increasing idle speed to 2,000 RPM produced about 8" of suction. If I bring the engine up to 3500 RPM, then quiclkly close the throttle, the vacuum spikes briefly to 10 inches. I then pulled the check valve out of the booster, and it did have stored vacuum in it. I unbolted the master cylinder, and moved it forward away from the booster, just to check the pedal action and resistance of the booster. Almost zero effort required to move the pedal to the floor. And the pushrod from the booster extends far enough out to ensure full travel of the M.C. pistons. So I am thinking the booster is fine, but it is simply not getting enough engine vacuum, because of the long-duration cam. Wondering if I should switch over to a manual-brake (non-power) master cylinder? Although I would still think that the 10-11" of vacuum produced during deceleration should be enough to provide some power braking effect from the booster?:confused:

I put a vacuum gauge on the manifold port, and at idle, it only creates 3-5" of vac @ 1200 RPM. Increasing idle speed to 2,000 RPM produced about 8" of suction. If I bring the engine up to 3500 RPM, then quiclkly close the throttle, the vacuum spikes briefly to 10 inches. I then pulled the check valve out of the booster, and it did have stored vacuum in it. I unbolted the master cylinder, and moved it forward away from the booster, just to check the pedal action and resistance of the booster. Almost zero effort required to move the pedal to the floor. And the pushrod from the booster extends far enough out to ensure full travel of the M.C. pistons. So I am thinking the booster is fine, but it is simply not getting enough engine vacuum, because of the long-duration cam. Wondering if I should switch over to a manual-brake (non-power) master cylinder? Although I would still think that the 10-11" of vacuum produced during deceleration should be enough to provide some power braking effect from the booster?:confused:

you need 15 inches of vacuum .. you have a big leak somewhere ..

you need 15 inches of vacuum .. you have a big leak somewhere ..

I guess one might consider it a massive vacuum leak, but I believe it is un-avoidable with this car/engine. It is caused by a combination of the substantial valve overlap from the long-duration/high-lift camshaft I am running, (Solid/mechanical flat-tappet, Lift .637", Duration 312*/322*, (270* @ .050" lift). 13:1 Compression ratio. And an open plenum/single-plane high-rise intake manifold with a Holley 850 CFM Double-Pumper carb. Engines like this just don't build much vacuum. It is definitely not a street-driven car.
That is why I am thinking maybe I need to switch to a manual (Non-Power-Assist) Master Cylinder...:uhoh:

I guess one might consider it a massive vacuum leak, but I believe it is un-avoidable with this car/engine. It is caused by a combination of the substantial valve overlap from the long-duration/high-lift camshaft I am running, (Solid/mechanical flat-tappet, Lift .637", Duration 312*/322*, (270* @ .050" lift). 13:1 Compression ratio. And an open plenum/single-plane high-rise intake manifold with a Holley 850 CFM Double-Pumper carb. Engines like this just don't build much vacuum. It is definitely not a street-driven car.
That is why I am thinking maybe I need to switch to a manual (Non-Power-Assist) Master Cylinder...:uhoh:

you must have good vacuum perhaps not a solid level but with little vacuum how is the fuel being sucked out of the carb ?

the brake booster vacuum line should have a check valve. the booster itself is a big tank to hold/smooth out the vacuum pulsations. the booster should have the higher vacuum with that check valve. if the check valve leaks/and or the booster leaks , then you won't.

placing your vacuum pump on the booster is a must do test with the proper components that do not leak. also checking all those vacuum lines for defects using the vacuum gauge and properly measuring for vacuum losses on each line including the carb vac operated pull offs..

its been a few decades since I messed with these vintage models but I also remember that the carb base gasket is also a big leak issue ..